Myeloperoxidase exerts an antimicrobial effect when combined with H2O2 and an appropriate oxidizable substance such as a halide, and this antimicrobial system appears to play an important role in neutrophilic polymorphonuclear leukocytes. The superoxide anion and hydroxyl radical, as well as a number of oxygen independent antimicrobial systems, also contribute to the microbicidal activity of the cell. We propose to study the properties, mechanism of action and biological significance of the oxygen-dependent antimicrobial systems with particular regard to the role of singlet molecular oxygen. The long term goals are: 1. to determine the role of these antimicrobial systems in the killing of microorganisms by neutrophils; 2. to determine whether alterations in the componenets of these systems might contribute to the pathogenesis of disease; and 3. to determine whether manipulations of these systems might influence the course of disease.